The past few years has witnessed the ever-increasing availability of relatively cheap, low power wireless data communication services, networks and devices, promising near wire speed transmission and reliability. One technology in particular, described in the IEEE Standard 802.11b-1999 Supplement to the ANSI/IEEE Standard 802.11, 1999 edition, collectively incorporated herein fully by reference, and more commonly referred to as “802.11b” or “WiFi”, has become the darling of the information technology industry and computer enthusiasts alike as a wired LAN/WAN alternative because of its potential 11 Mbps effective data transmission rate, ease of installation and use, and transceiver component costs make it a real and convenient alternative to wired 10 BaseT Ethernet and other cabled data networking alternatives. With 802.11b, workgroup-sized networks can now be deployed in a building in minutes, a campus in days instead of weeks since the demanding task of pulling cable and wiring existing structures is eliminated. Moreover, 802.11b compliant wireless networking equipment is backwards compatible with the earlier 802.11 1M/2 Mbps standard, thereby further reducing deployment costs in legacy wireless systems.
802.11b achieves relatively high payload data transmission rates through the use of orthogonal class modulation in general, and, more particularly, 8-chip complementary code keying (“CCK”) at a 11 MHz chipping rate. As such, bitstream data is mapped into nearly orthogonal sequences (or code symbols) to be transmitted, where each chip of the code symbol is quaternary phase modulated. An 802.11b compliant receiver correlates the received CCK modulated signal with 64 candidate waveforms to find the most likely code symbol, from which the bitstream data is recovered through reverse mapping. The high-rate physical layer PLCP preamble and header portions are still modulated using the 802.11 compliant Barker spreading sequence at an 11 MHz chipping rate, resulting in a 1 or 2 Mbps effective header and preamble transmission rate depending on whether DBPSK or DQPSK modulation is employed.
CCK was chosen in part because of its strong inherent resistance to multipath interference, which is likely to be encountered in the typical in-building deployment. Nevertheless, the confluence of strict power/noise limits specified for operation in the 2.4 GHz ISM band and megabit+ expected data throughput rates limits conventional 802.11b to just a 100 or so feet between stations, depending on the number of interposing radio obstructions and reflections.
Thus 802.11b remains susceptible to multipath interference, and to reception errors produced by inter-symbol (“ISI”) and inter-chip interference (“ICI”) in particular. To combat this, designers have sought to improve receiver performance, at least with respect to CCK code symbol demodulation by using active equalization techniques. However, such techniques do not appear to take into account symbol processing reliability or errors, much less track such errors, nor alter the transmission environment when post symbol processing signal quality degrades.